1. Field
The invention relates generally to the separation of liquids and solids by means of plate type pressure filters, specially filter presses utilizing center feed filter chambers. More particularly, this invention relates to an improved core blow process performed after a completed filtration cycle.
2. Description of the Art
The filter press apparatus has been used for more than one hundred years and is well known in the art. Filter presses are described in numerous United States Patents. See, for example, U.S. Pat. No. 4,293,417. The filter press is used to separate solids from a mixture of solids and liquid, or slurry. These devices are used in various industries including, for example, mining operations and sewage treatment.
A filter press generally includes a plurality of separable plates. These plates are slidably held on a frame structure between a fixed support and an opposing moveable support. The moveable support, known generally as a follower, is actuated typically by a hydraulic ram. A simple manual screw and capstan may be used, however, for small filter presses.
The filter plates are typically arranged in parallel about a horizontal axis. In order to effect filtering, the row of plates are compressed one against another by, for example, a horizontally mounted hydraulic ram. The compression of the plates, one against another, serves to form a sealed abutment of each plate against its neighboring plates. Filter cloths are typically suspended between the cavities defined by the adjacent filter plates, or caulked into grooves formed in the individual filter plates. Each filter plate contains a hole in the recessed portion of the plate, thereby defining a horizontal feed core through the stack of filter plates. This type of plate, having a hole in the recessed portion of the plate, is typically referred to as a center feed plate, though the term is somewhat of a misnomer since the hole need not be in the center of the plate. A filter liner plate is placed at each end of the row of filter plates. The filter liner plate adjacent to the follower is typically referred to as the follower liner plate.
A feed stream is fed through the core under pressure, whereby the slurry is forced through the cavities between filter plates. The solids within the stream are retained by the filter cloth as the stream passes through the cloth. After the cloths become sufficiently laden with solid deposits, efficient operation of the press requires the removal of the cakes formed from the solid deposits retained by the filter cloths.
After the feed stream is halted, remnants of slurry from the feed stream typically remain in the feed core. Any slurry remaining in the core can cause significant problems. When the filter press opens and the filter cakes are discharged any remaining feed core slurry will run down the plate surfaces and may build up on stay bosses and lower sealing surfaces. This cake build up, when the press is again closed and pressurized, may cause structural damage to the filter plates or even to the filter press itself. In addition, liquid remaining in the feed core, which subsequently seeps into the cakes, increases the moisture content of the cakes. Minimal cake moisture is essential to avoid sticking to conveyor belts or other materials handling equipment, and where the solids must be transported significant distances. Finally, significant labor is required to remove the slurry remaining in the core.
To mitigate the problems associated with the slurry remnants in the core, a core blow process is typically performed whereby air is blown at high pressure through the core in the opposite direction of the feed stream. This process, typically performed shortly after the feed stream is halted, is used to remove the slurry remaining in the core. The core blow process is typically performed by two methods, depending primarily on the type of ram used in the filter press.
In filter presses utilizing a ram press wherein the ram arm does not press against the follower on the opposite side of the feed core, a horizontal, and typically cylindrical, passageway along the axis of the feed core is formed through the follower liner plate and the follower. After the filtration cycle is finished, a pressurized air supply is then attached to this passageway to force the remnants of slurry back out through the entrance of the slurry feed. This procedure is termed a horizontal-feed core blow process.
In other ram presses, whereby the ram arm presses against that portion of the follower on the opposite side of the feed core, a horizontal passageway through the follower and leading into the feed core cannot be formed. To avoid interference with the ram arm, a vertical passageway is formed between the top surface of the follower liner plate and a horizontal chamber formed partway through the follower liner plate leading into the feed core. A pressurized air source is then attached to the top surface of the follower liner plate whereby the pressurized air is directed into the vertical passageway, through the horizontal chamber, and into the feed core. This procedure is termed a vertical-feed core blow process.
In both types of filter presses, the core blow process is ineffective for removing all of the slurry remnants in the feed core. This ineffectiveness results in part from the fact that the pressurized air line is smaller in diameter than the feed core. Practical limitations restrict the size of the pressurized air line, particularly in those filter presses where the pressurized air source is attached to the top surface of the follower liner plate, whereby the pressurized air line must be smaller than the width of the follower liner plate.
Because of the relatively large core diameters compared to the compressed air feed lines used in the core blow process, much of the slurry remaining in the feed core is untouched by the pressurized air source. The previously utilized core blow process is therefore ineffective for removing the remnants of slurry in the feed core.
A more effective means of removing the slurry remaining in the feed core is therefore essential to: (1) provide uniform plate sealing; (2) prevent uneven plate stress from residue cake build-up; (3) prevent premature wear of the filter press resulting from uneven plate stress; (4) ensure minimal cake moisture; and (5) reduce maintenance and cost by extending intervals between plate washings.